Interest in terahertz frequency systems has been steadily increasing over the past several years as their unique capabilities are identified and technological barriers to their realization are overcome. The terahertz frequency region (frequencies of ca. 0.3 THz and higher (corresponding to wavelengths of ca. 1 mm and shorter) offers a number of advantages including large bandwidths and a significant reduction in size and weight relative to existing microwave electromagnetic systems. Several applications that depend on or are expected to benefit from terahertz frequency technology have been proposed or are under development. These applications include spectroscopic detection of atmospheric pollutants (e.g. ozone, greenhouse gases) and chemical warfare agents, high resolution imaging systems for low visibility environments and optically opaque media (e.g. fog, smoke, night, concealed object detection), short-range communications systems and satellite crosslinks. Military applications of terahertz frequency systems include aircraft guidance systems, portable radars, missile seekers, and battlefield communications. Possible civilian applications include automotive collision avoidance systems, blind-spot indicators, freeway tolling, product tagging, and wireless communications.
Realization of terahertz frequency systems requires the development of new technologies for the transmission, manipulation and reception of electromagnetic radiation in the millimeter to sub-millimeter wavelength portion of the electromagnetic spectrum. For convenience, the millimeter to sub-millimeter wavelength portion of the electromagnetic spectrum may be referred to herein as the millimeter wavelength range. Traditional approaches have emphasized extensions of electronic technologies used in the radiofrequency (rf) range (frequencies from about 3 MHz to about 30 GHz, wavelengths from about 10 m to about 1 cm) to the terahertz frequency range. Electromagnetic radiation in the rf region has been utilized for many years in radar imaging and tracking systems and has a well-developed base of technology that includes elements for transmitting, steering, focusing and receiving rf radiation. Conventional rf technology is an electronic circuit based technology in which discrete electronic components such as capacitors and inductors are used to form transmission lines, waveguides, and phase shifters that are used to directly produce, control and detect rf radiation. Although conventional rf systems are generally compatible with millimeter wave applications, problems associated with cost, power and functionality arise, especially in the sub-millimeter range. Cost issues stem from the expensive phase shifters needed for the directional transmission or reception of rf radiation. Power issues arise in the context of active phase arrays, which utilize solid-state devices. The power handling capability of solid-state devices is frequency dependent and decreases with increasing frequency of electromagnetic radiation. The output power of a typical Si bipolar junction transistor, for example, decreases by over three orders of magnitude when the frequency is changed from 1 GHz to 30 GHz. Moreover, at sufficiently high frequencies, discrete electronic components (e.g. capacitors and inductors) lose their functionality.
Recent developments in millimeter wave systems have emphasized spatially based systems in which the radiation propagates in free space, rather than through circuits, waveguides or transmission lines. Spatially based systems are also referred to as quasi-optical systems because they treat millimeter wave radiation as free space beams and manipulate these beams in a manner analogous to methods used for manipulating optical beams. Quasi-optical elements can be used to amplify, mix, switch, reflect, transmit or phase shift millimeter wave beams. Quasi-optical elements can be passive or active. A passive quasi-optical element typically consists of a grid of a conducting material. The grid can be formed, for example, from a thin sheet of a conducting material that includes periodically spaced perforations. Wire grids are also possible. The grids can be free standing, mounted on a dielectric substrate, or sandwiched between two dielectric materials. Active quasi-optical elements incorporate a periodic array of active (non-linear) devices into an otherwise passive grid. The active devices can be placed at the intersection points of a grid or between intersection points. Active devices such as PIN diodes, varactor diodes, transistors, polarizers, amplifiers, transducers, and Faraday rotation devices have been used in active quasi-optical elements. Quasi-optical elements offer the potential of overcoming the power inadequacies of circuit based millimeter wave systems by providing an effective way to combine the outputs of multiple solid state devices to achieve high powers.
Interest in quasi-optical elements for millimeter wave systems has steadily increased over the past decade as methods for producing monolithic two-dimensional periodic arrays of active devices have improved and fundamental principles of operation have been elucidated. There remain, however, several limitations that require attention in order for quasi-optical elements to become mainstream. Outstanding limitations include active element insertion losses, scattering losses, active element power requirements, thermal effects due to power dissipation, and manufacturing challenges for large-scale grids such as precise substrate flatness requirements and precise dielectric thickness uniformity. Further performance improvements (e.g. greater variability of phase angle, wider reflection angles for stationary elements, tighter beam focusing, lower power requirements and/or heat dissipation in active grids) are also desired. A need exists for new quasi-optical materials and elements to further the potential of quasi-optical systems for use in controlling electromagnetic radiation, especially in the terahertz frequency portion of the electromagnetic spectrum.